Lattices are commonly used as light-weight structures due to their inherent cavities. Examples of these structures are truss bridges, stadiums' framework roofs and telescope supporters. In the simple two-dimensional (2D) space, the common periodic lattices are constructed from the geometrical shapes of regular polygons such as equilateral triangle, square and regular hexagon. See FIG. 1 (Ashby and Gibson, 1997; Fleck el al., 2010).
Nevertheless, in some cases, the mechanical properties of the lattices such as tensile strength, hardness, or ductility are not able to fulfill the requirements in certain applications.
Stainless steel 316L is a traditional material that can be utilized for fabricating cardiovascular stents due to an excellent combination of mechanical properties, corrosion resistance and biocompatibility. However, in comparison with some other metallic biomaterials for stents (e.g. cobalt chromium alloy, Co—Cr), 316L SS is still inferior in terms of yield strength and hardness, hence the strut thickness of 316L SS stents (˜150 μm) should be much thicker than that of Co—Cr stents (˜90 μm) to meet the mechanical requirements. Metallic stents are foreign matters to human body, the targeted vessel could be re-narrowing after the long-term intervention due to the adverse tissue reactions such as inflammations and immunological rejections. It is demonstrated by patient outcomes that stents with thicker struts result in higher restenosis rates compared to those with thinner struts. Moreover, the thick struts of stents will compromise the flexibility, thus the track of stents through the guide catheter and through the tortuous anatomy of the coronary arteries will be more difficult. There are other problems in existing metallic stents such as potential toxic Ni release, relatively high cytotoxicity, low cytocompatibility to certain cell type (e.g. endothelial cells), and low hemocompatibility.
CN101899554A disclosed a NiTi alloy which is treated with plasma nitriding followed by surface mechanical attrition treatment (SMAT) to improve the hardness of the NiTi alloy. Although plasma nitriding treatment was shown to improve the hardness of NiTi alloy in CN101899554A, plasma nitriding will cause unwanted effects on other metallic alloys, especially stainless steel because of the high content of iron in stainless steel which becomes unstable after plasma nitriding. Plasma nitriding also increase the Ni release from those metallic alloys, which is unfavorable to the cell growth and tissue regeneration around an implantable medical device such as stent made of those metallic alloys.
Therefore, a 316L SS alloy with a higher yield strength and hardness, reduced Ni release, relatively lower cytotoxicity, higher cytocompatibility to endothelial cells, and improved hemocompatibility as an ideal biomaterial for fabricating implantable medical device is needed.